1. Field of the Invention
The present invention relates generally to a cellular Code Division Multiple Access (CDMA) communication system. More particularly, the present invention relates to a method and apparatus for transmitting and receiving downlink control information in the case where an Enhanced Uplink Dedicated transport CHannel (E-DCH) is used.
2. Description of the Related Art
A 3rd generation mobile communication system using WCDMA based on the European Global System for Mobile communications (GSM) system and General Packet Radio Services (GPRS), Universal Mobile Telecommunication Service (UMTS) provides mobile subscribers or computer users with a uniform service of transmitting packet-based text, digitized voice, and video and multimedia data at or above 2 Mbps irrespective of their locations around the world.
In particular, the UMTS system uses a transport channel called the E-DCH in order to further improve the packet transmission performance of uplink communications from a User Equipment (UE) to a Node B (interchangeable with a base station). For more stable high-speed data transmission, Adaptive Modulation and Coding (AMC), Hybrid Automatic Repeat reQuest (HARQ), Node B-controlled scheduling, and shorter Transmission Time Interval (TTI) were introduced for the E-DCH transmission.
AMC is a technique of determining a Modulation and Coding Scheme (MCS) adaptively according to the channel status between the Node B and the UE. Many MCS levels can be defined according to available modulation schemes and coding schemes. The adaptive selection of an MCS level according to the channel status increases resource use efficiency.
HARQ is a packet retransmission scheme for retransmitting a packet to correct errors in an initially transmitted packet. HARQ is branched into Chase Combining (CC) and Incremental Redundancy (IR). The HARQ scheme adopts N-channel Stop and Wait (SAW) to increase data rate. In the N-channel SAW HARQ, a transmitter transmits different data in first to Nth Transmission Time Intervals (TTIs), and determines whether to retransmit the data or transmit new data in (N+1)th to 2Nth TTIs according to Acknowledgement/Non-Acknowledgement (ACK/NACK) received for the transmitted data. N TTIs are processed by separate HARQ processes and each of HARQ processes for the (N+1)th to 2Nth TTIs is called an ith HARQ process. N is an integer greater than 0 and the HARQ process number i is an integer number ranging from 1 to N.
Node B-controlled scheduling is a scheme in which the Node B determines whether to permit E-DCH transmission for the UE and if it does, an allowed maximum data rate and transmits the determined data rate information as a scheduling grant to the UE, and the UE determines an available E-DCH data rate based on the scheduling grant.
Shorter TTI is a technique for reducing retransmission time delay and thus increasing system throughput by allowing the use of a shorter TTI than the shortest TTI of 10 ms provided by 3GPP Rel5.
FIG. 1 illustrates uplink packet transmission on the E-DCH in a typical wireless communication system.
Referring to FIG. 1, reference numeral 100 denotes a Node B supporting the E-DCH and reference numerals 101 to 104 denote UEs using the E-DCH. As illustrated, the UEs 101 to 104 transmit data to the Node B 100 on E-DCHs 111 to 114.
The Node B 100 notifies the individual UEs 101 to 104 whether they are permitted for E-DCH transmission or transmits to the UEs scheduling grants indicating E-DCH data rates for them, based on information about buffer occupancy and requested data rates or channel status information received from the UEs. This operation is called scheduling of uplink data transmission. The scheduling is performed such that the noise rise or Rise over Thermal (ROT) measurement of the Node B does not exceed a target ROT to increase total system performance by, for example, allocating low data rates to remote UEs (such as the UEs 103 and 104) and high data rates to nearby UEs (such as the UEs 101 and 102). The UEs 101 to 104 determine their allowed maximum data rates for E-DCH data based on the scheduling grants and transmit the E-DCH data at the determined data rates.
Due to asynchronization between uplink signals from different UEs, the uplink signals interfere with one another. As the Node B receives more uplink signals, an uplink signal from a particular UE suffers from increased interference, thereby decreasing reception performance in the Node B. This problem can be overcome by increasing the uplink transmit power of the UE, but the increased transmit power in turn serves as interference to other uplink signals. Thus, the reception performance is still decreased in the Node B. The total power of uplink signals that the Node B can receive with reception performance at or above an acceptable level is limited. ROT represents uplink radio resources used by the Node B, defined asROT=Io/No  (1)where Io denotes power spectral density over a total reception band, that is, the total amount of uplink signals received in the Node B, and No denotes the thermal noise power spectral density of the Node B. Therefore, an allowed maximum ROT is total uplink radio resources available to the Node B.
The total ROT is expressed as the sum of inter-cell interference, voice traffic and E-DCH traffic. With Node B-controlled scheduling, simultaneous transmission of packets from a plurality of UEs at high data rates is prevented, maintaining the total ROT at or below a target ROT and thus ensuring reception performance all the time. When high data rates are allowed for particular UEs, they are not allowed for other UEs in the Node B-controlled scheduling. Consequently, the total ROT does not exceed the target ROT.
FIG. 2 is a diagram illustrating a typical signal flow for message transmission and reception on the E-DCH.
Referring to FIG. 2, a Node B and a UE establish an E-DCH in step 202. Step 202 involves message transmission on dedicated transport channels. The UE transmits scheduling information to the Node B in step 204. The scheduling information may contain uplink channel status information including the transmit power and power margin of the UE, and the amount of buffered data to be transmitted to the Node B.
In step 206, the Node B monitors scheduling information from a plurality of UEs to schedule uplink data transmissions for the individual UEs. The Node B decides to approve an uplink packet transmission from the UE and transmits a scheduling grant to the UE in step 208. The scheduling grant indicates up/hold/down in an allowed maximum data rate, or an allowed maximum data rate and an allowed transmission timing.
In step 210, the UE determines the TF of the E-DCH based on the scheduling grant. The UE then transmits TF information to the Node B, and uplink packet data on the E-DCH at the same time in steps 212 and 214. The TF information includes a Transport Format Resource Indicator (TFRI) indicating resources required for E-DCH demodulation. The UE selects an MCS level according to an allowed maximum data rate set by the Node B and its channel status, and transmits the E-DCH data in step 214.
The Node B determines whether the TF information and the uplink packet data have errors in step 216. In the presence of errors in either of the TF information and the uplink packet data, the Node B transmits a NACK signal to the UE on an ACK/NACK channel, whereas in the absence of errors in both, the Node B transmits an ACK signal to the UE on the ACK/NACK channel in step 218. In the latter case, the packet data transmission is completed and the UE transmits new packet data to the Node B on the E-DCH. On the other hand, in the former case, the UE retransmits the same packet data to the Node B on the E-DCH.
Under the above-described environment, if the Node B can receive from the UE scheduling information including, for example, information about the buffer occupancy and power status of the UE, it allocates a low data rate to the UE if it is far from the Node B, is in a bad channel status, or has data of a lower service class. If the UE is near to the Node B, is in a good channel status, or has data of a higher service class, the Node B allocates a high data rate to the UE. Therefore, the total system performance is increased.
In the case where the Node B transmits a Relative Grant (RG) indicating up/hold/down in the allowed maximum data rate of the UE as a scheduling grant for the E-DCH, the signaling overhead of the RG reduces downlink capacity. Accordingly, a need exists for a method of reducing downlink signaling overhead arising from transmitting a scheduling grant in Node B-controlled scheduling.